1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having a good image quality and yield.
2. Discussion of the Related Art
Recently, thin film transistor liquid crystal display devices (TFT LCD) have been used as display devices for portable televisions and notebook computers, etc. However, the conventional TFT LCDs have angular dependence problems.
In order to overcome this angular dependence problem, a twisted nematic LCD having an optical compensation plate and a multi-domain LCD has been introduced. However, since the contrast ratio in these LCDs is dependent upon the viewing angle, the color of the image is shifted.
FIG. 1 is a plan view of a conventional liquid crystal display device. As shown in FIG. 1, a pixel is defined by a gate bus line 3 and a data bus line 4. Although only one pixel is drawn in FIG. 1, the real liquid crystal display device has a plurality of pixels. At the intersection point between the gate bus line 3 and the data bus line 4, a thin film transistor (TFT) is located with a gate electrode 5 connected to the gate bus line 3 and source/drain electrodes 6 connected to the data bus line 4.
FIG. 2 is a sectional view taken along the line I-I' of FIG. 1. The LCD includes a TFT array substrate 1 and a color filter substrate 2. As shown in FIG. 2, the data bus line 4 and a transparent pixel electrode 7 are formed on an insulating layer 11 deposited over the TFT array substrate 1, with a passivation layer 13 and a first alignment layer 14a formed thereon. On the color filter substrate 2, a color filter layer 10, and a black matrix 8 for preventing the light leakage through the TFT, the gate bus line 3, and the data bus line 4, are formed. A transparent counter electrode 17 is also formed over the color filter substrate 2, and a passivation layer 12 is deposited thereon. A second alignment layer 14b is coated on the passivation layer 12.
When a voltage is not applied to the pixel and the counter electrode 17 and the pixel electrode 7, the liquid crystal molecules in a liquid crystal layer 19 sandwiched between the TFT array substrate 1 and the color filter substrate 2 are arranged along the alignment direction of the first and second alignment layers 14a and 14b. By applying a voltage, an electric field is generated between the pixel electrode 7 and the counter electrode 17 so that the liquid crystal molecules are arranged perpendicular to the surface of the substrates 1 and 2. Thus, the angular dependence problem is generated because of the refractive anisotropy of the liquid crystal molecules, and the color is shifted.
To obtain a wide viewing angle, in-plane switching LCDs are disclosed in JAPAN DISPLAY 92 (P547), Japanese Patent Unexamined Publication No. 7-36058, Japanese Patent Unexamined Publication No. 7-225538, and ASIA DISPLAY 95 (P707).
FIGS. 3 and 4 illustrate a conventional in-plane switching liquid crystal display device. First and second alignment layers 14a and 14b are rubbed in a slightly oblique direction relative to the extension direction of the data bus line 4 to align the liquid crystal molecules in a certain direction. A polarizer (not shown in the drawing) having a polarization axis parallel to the extension direction of the gate bus line 3 is attached to the TFT array substrate 1 and an analyzer having a polarization axis parallel to the rubbing direction is attached to the color filter substrate 2. In the pixel, at least one data electrode 21 and one common electrode 22 are extended in the perpendicular direction to the extension direction of the gate bus line 3. When the voltage is not applied to the data electrode 21 and the common electrode 22, the liquid crystal molecules of the liquid crystal layer 19 are aligned in the slightly oblique direction relative to the extension direction of the electrodes 21 and 22 along the rubbing direction. When the voltage is applied, an electric field parallel to the extension direction of the gate bus line 3 is generated in the liquid crystal layer 19 to switch the liquid crystal molecules in the plane level of the surfaces of the substrates 1 and 2. Accordingly, the viewing angle characteristics and the contrast ratio are improved because of the in-plane switching of the liquid crystal molecules.
However, there are several problems in the conventional in-plane switching liquid crystal display device. The liquid crystal molecules are easily polarized by an outer induced electric field. Especially, the liquid crystal molecules in the liquid crystal layer are mainly polarized by the outer induced electric field through the color filter substrate during an user's application. The most general outer induced electric field is a field caused by a human hand approaching the screen. The electrostatic charges associated with the human hand cause the polarization of the liquid crystal molecules. As a result, a spot is generated on the screen.